Young stars and planets grow up together

Flora Malein, 3rd January 2013

Astronomers have shed further light on the mysterious process of how young stars and their planetary systems form.

The ALMA telescope in the Atacama Desert in Chile, is helping scientists to unlock new mysteries about how solar systems form. Astronomers using ALMA have been observing the formation of a young star and its planets around 450 light years away.

While it is understood how the young planets grow at this time, pulling in matter from the giant dust clouds which surround them, until now scientists have not known how the young star also continues to grow at this time.

As the young, ‘proto-planets’ pull in matter by their gravitational force, the incoming material forms a flat spinning disk around the star. This process creates a gap between the young star and the main body of gas, making it apparently impossible that the young star has access to enough matter to grow at this time.

"This has been a bit of a mystery, but now we have found a process that allows the star to continue to grow despite the gap," says Simon Casassus, an astronomer at the University of Chile who led the international research team using ALMA.

By observing the young system, known as ‘HD 142527’, currently going through this process, the team discovered that the gap is not empty like they had once believed but filled with a thin layer of carbon monoxide gas.

"Whereas dust is severely depleted within the gap, some residual gas remains," says Gerrit van der Plas, one of the study’s co-authors.

ALMA also revealed the presence of ‘streamers’ of dense gas made up of the ion HCO+ that appear to flow round the planets and close to the star. These streamers, probably caused by the gravitation pull of the young planets, are what bring matter close enough for the young star to feed off.

"The most natural interpretation for the flows seen by ALMA is that the putative proto-planets are pulling streams of gas inward toward them that are channelled by their gravity. Much of the gas then overshoots the planets and continues inward to the portion of the disk close to the star, where it can eventually fall onto the star itself," says Casassus.

A computer simulation of the process revealed that the streamers would be enough to maintain the inner disk and the star's rate of growth, providing the first explanation for how the star sustains itself despite its separation.

The distances involved in the HD 142527 system are on a greater scale than our own solar system: the baby planets have not been observed directly as they are obscured by dense gas clouds but the team believe that their size is several time greater than Jupiter, the largest planet in our solar system.